Dark Energy Survey: implications for cosmological expansion models from the final DES Baryon Acoustic Oscillation and Supernova data

The Dark Energy Survey (DES) recently released the final results of its two principal probes of the expansion history: Type Ia Supernovae (SNe) and Baryonic Acoustic Oscillations (BAO). We explore the cosmological implications of these data in combination with external Cosmic Microwave Background (CMB), Big Bang Nucleosynthesis (BBN), and age-of-the-Universe information. The BAO measurement, $\sim2σ$ away from Planck’s $Λ$CDM predictions, pushes for low values of $Ω_{\rm m}$ compared to Planck, in contrast to SN which prefers a higher value. We identify several tensions among datasets in the $Λ$CDM model that cannot be resolved by including either curvature or a constant dark energy equation of state. By combining BAO+SN+CMB despite these mild tensions, we obtain $Ω_k$=$-5.5^{+4.6}{-4.2}\times10^{-3}$ in $kΛ$CDM, and $w=-0.948^{+0.028}{-0.027}$ in $w$CDM. In $w$CDM, BAO and SN push again in different directions of parameter space, favoring, respectively $w<-1$ and $w>-1$. If we open the parameter space to $w_0w_a$CDM, all the datasets are mutually more compatible, and we find concordance in the $w_0>-1,w_a<0$ quadrant, with BAO pushing for $w_a<0$ and SN for $[w_0>-1,w_a<0]$. For DES BAO and SN in combination with Planck-CMB, we find a $3.2σ$ deviation from $Λ$CDM, with $w_0=-0.673^{+0.098}{-0.097}$, $w_a = -1.37^{+0.51}{-0.50}$, a Hubble constant of $H_0=67.81^{+0.96}{-0.86}$km s$^{-1}$Mpc$^{-1}$, and an abundance of matter of $Ω{\rm m}=0.3109^{+0.0086}_{-0.0099}$. For the combination of all the background cosmological probes considered we still find a deviation of $2.8σ$ from $Λ$CDM in the $w_0-w_a$ plane. Assuming a minimal neutrino mass, this work provides tentative evidence for non-$Λ$CDM physics, which is consistent with recent claims in support of evolving dark energy, or a source of unknown systematics.

💡 Research Summary

The Dark Energy Survey (DES) has released its final measurements of two key background probes of cosmic expansion: Baryon Acoustic Oscillations (BAO) from a sample of ~16 million galaxies spanning 0.6 < z < 1.2, and Type Ia Supernovae (SN) from 1,635 DES‑discovered SNe Ia plus 194 low‑z external events. The BAO analysis yields a distance‑to‑sound‑horizon ratio D_M/r_d at an effective redshift z_eff = 0.85 that is about 2 σ lower than the prediction of the Planck ΛCDM model, implying a lower matter density Ω_m. In contrast, the SN Hubble diagram prefers a higher Ω_m when interpreted with the same CMB baseline. This opposite pull creates a tension that cannot be fully resolved within the standard flat ΛCDM framework (Ω_k = 0, w = ‑1).

The authors therefore explore three extensions: (i) curvature (kΛCDM), (ii) a constant dark‑energy equation of state (wCDM), and (iii) a time‑varying equation of state parameterized by the CPL form w(a)=w₀+w_a(1‑a). Combining BAO, SN, and external data (Planck temperature‑polarization spectra, BBN, and an age‑of‑the‑Universe prior) they find:

• In kΛCDM, Ω_k = ‑5.5 × 10⁻³ ± 4.5 × 10⁻³, consistent with flatness but not enough to reconcile the BAO–SN discrepancy.
• In wCDM, the joint fit yields w = ‑0.948 ± 0.028, a modest deviation from the cosmological‑constant value; BAO pushes w < ‑1 while SN pushes w > ‑1.
• In the CPL w₀w_aCDM model, the data become mutually more compatible. The combined DES BAO+SN+Planck analysis gives w₀ = ‑0.673 ± 0.10 and w_a = ‑1.37 ± 0.51, a 3.2 σ departure from ΛCDM (w₀ = ‑1, w_a = 0). The corresponding Hubble constant is H₀ = 67.81 ± 0.90 km s⁻¹ Mpc⁻¹ and the matter density Ω_m = 0.3109 ± 0.009.

When all background probes considered in the paper (including the CMB acoustic scale θ_* and a direct age constraint) are combined, the deviation from ΛCDM in the w₀–w_a plane remains at 2.8 σ. The analysis assumes a minimal neutrino mass (∑m_ν ≈ 0.06 eV); allowing larger neutrino masses does not qualitatively change the conclusions.

The authors interpret these results as tentative evidence for physics beyond ΛCDM. The preferred region lies in the quadrant w₀ > ‑1, w_a < 0, suggesting an evolving dark‑energy component that becomes less negative at late times. Alternatively, the persistent tension could arise from unknown systematic effects in either the BAO measurement (e.g., sound‑horizon calibration, photometric redshift uncertainties) or the SN distance ladder (e.g., absolute‑magnitude calibration, selection biases).

In summary, the final DES BAO and SN data, when confronted with the most precise CMB, BBN, and age information, reveal a statistically significant inconsistency with a static‑w ΛCDM universe. This motivates further investigations with independent growth‑of‑structure probes (weak lensing, galaxy clustering, cluster counts, redshift‑space distortions) and upcoming surveys such as DESI, Euclid, and the Rubin Observatory, which will be essential to confirm whether the signal points to genuine dark‑energy dynamics or to yet‑unidentified systematic errors.